Now not knowing if you want to snipe or shoot then I can offer the following advice. DO NOT SNIPE TO START WITH, it is not like a PC game or real life sniping, there is no 1 mile kills. Sniper rifles have 1 shot and can shoot a LITLE bit further than a well set up assault rifle.

So based upon those sage words of sdvice to a starter, and also having no idea as to what type of site you will be playing at, be it woodland or close quarters, I would recomend the following

ics m4 transparent

g&g m4 or ak47 two tone

both are around 300-320 fps, so within both site limits, both are two tone, both can be bought in the UK

however a bit more info would be appreciated for help (also try to se the search function or scroll down a bit as it is asked over and over again)